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On Sep 21, 2012, at 5:18 PM, Alan McKinnon <email@example.com> wrote:
> On Fri, 21 Sep 2012 15:52:17 -0400
> Michael Mol <firstname.lastname@example.org> wrote:
>> On Fri, Sep 21, 2012 at 3:45 PM, Alan McKinnon
>> <email@example.com> wrote:
>>> On Fri, 21 Sep 2012 15:25:49 -0400
>>> Michael Mol <firstname.lastname@example.org> wrote:
>>>> Guys, this fell into politics, one of those categories of things
>>>> you don't discuss in polite company. Not that I'm accusing anyone
>>>> of being polite,
>>> Phew, glad to hear that last bit. You had me worried for a second,
>>> what with my reputation to uphold and all
>>>> Now, could we go back to discussing software, packages, electronics
>>>> and amplifiers? I found that portion of the thread utterly
>>> That's a good idea. Would you like to hear about Hitachi Class H
>>> I'm forever fascinated that I seem to be the only person that ever
>>> heard of them. Most techies know A, AB and B. Some know Class C but
>>> I get blank looks everywhere I mention Class H...
>> I would indeed. And a primer (or reasonable reference for someone with
>> just a technician's amateur radio license) on class C.
> IIRC this was back in the late 70s or early 80s. Someone at Hitachi
> figured that amps (like code) spent 90% of their time doing 10% of the
> effort. If you had a 100W amp, it wasn't trying to drive 100W into the
> speakers all the time - only when the input signal was large enough.
> And yet, the power source for the output stages was permanently running
> at 70V or so (that's what it takes to get 100W into speaker coils back
> then). A transistor isn't a perfect isolator when biased off, so some
> of that voltage gets dropped somewhere (across the output transistors)
> and the result is a lot of wastage.
> wikipedia has a quite good summary of the usual classes - A, B, AB, C &
> But, Class H. A Hitachi engineer had a brilliant idea:
> Run the damn thing class A all the time (for the audio quality) but at
> around 24V. Heat generated is minimal. The power supply had a fancy
> voltage tripler circuit and when the input warranted it, the supply
> voltage would (very rapidly) switch over to the full 70V and the amp
> would deliver the full rated output. There was fancy circuitry in place
> to avoid distortion at the switch on point of course, but that is a bit
> An interesting take on the problem. Mechanical engineers do this all
> the time with engines - turbos only kick in when you need the power
> boost they provide, the rest of the time the motor is in regular mode.
> I've promised myself for years since my apprentice days that I would
> one day built a valve amp from a kit. There's something about the warm
> glow from the tubes on a winter night that is appealing :-) I'd better
> hurry up and get on with it, I read that decent quality valves are
> becoming scarce and are generally only available from (what used to be)
> the USSR.
> Alan McKinnon